1. Field of the Invention
The invention relates to a method and a system for measuring stress and other parameters in a structure.
2. Description of Related Art
When a new aircraft is placed in service, the manufacturer estimates a useful life for the airframe. In order to determine the useful life, the manufacture designs, builds and tests structures for the spectrum of stresses expected to be encountered during the operation of the aircraft. The tests verify failure limits and fatigue damage rates for the structure and serve as a basis for a best estimate of the structural life with safety factors included.
When a large fleet of production aircraft is involved, the stresses and environment to which an individual airplane is subject varies widely. An airplane which has been subjected to a harsh stress environment will reach the end of its structural life thousands of flight hours before an aircraft which has been operated in a benign stress environment. Currently, fleet-wide Individual Aircraft Tracking (IAT) and Loads Environmental Survey systems are installed in DoD aircraft to monitor usage and provide data for fatigue life expenditure and potential crack growth calculations. Periodic inspections are required to determine the flight worthiness of each particular aircraft. Occasionally, these inspections result in an unsafe airplane being flown or a safe airplane being grounded. This method of fatigue life appraisal is labor intensive, requires substantial aircraft out of service time, and is a major contributor to the high life cycle costs of manned aircraft weapon systems.
As a method of avoiding these problems, "smart structures" have been proposed which typically consist of aircraft structural members with multiple embedded or attached sensor elements, and some method of collecting, analyzing and recording information regarding the structural integrity and stress history of the airframe. For smart structures to be relied on for mission or flight critical decisions, all essential flight critical structural elements must be monitored and integrity assessed in real time. Accomplishing this requires a large number of individual sensors with output measured and structural integrity assessed in real time. Current methods are only able to monitor a limited number of sensors with structural integrity assessments accomplished later by ground based computers.